1. Field of the Invention
The present invention relates to a dielectric resonator and a dielectric filter for use in a base station for mobile communication such as portable telephone, a transmitting station for broadcasting, and the like, and to a method of supporting a dielectric resonance element used in the dielectric resonator and the dielectric filter.
2. Related Art of the Invention
In recent years, high-sensitivity transmission/reception performance and good communication quality have become indispensable to portable telephone systems. It is, therefore, required that a filter for use in a base station should have a low-loss transmission characteristic such as to cause substantially no degradation in signal components and a sharp attenuation characteristic such as to reliably remove unnecessary interfering wave components. Also, there has been an increasing demand for reducing the size as well as improving electrical characteristics. An example of filters capable of meeting such a demand is a TM mode dielectric filter using a TM mode dielectric resonator of a high Q-value.
An example of a conventional dielectric resonator and a dielectric filter using the dielectric resonator will be described with reference to drawings. FIG. 9(a) is a cross-sectional view of a conventional TM mode dielectric resonator, FIG. 9(b) is a cross-sectional view of the conventional TM mode dielectric resonator taken along the line B–B′ in FIG. 9(a) and seen from a position above the resonator, and FIG. 10 shows an electromagnetic field distribution in the conventional dielectric resonator. The dielectric resonator has input/output terminals 701a and 701b, input/output probes 702a and 702b, a dielectric resonance element 703, a metallic casing 704, a metallic cover 705, connecting screws 706, and a frequency adjusting screw 707. Arrow 801 indicates an electric force line and arrow 802 indicates a magnetic force line. Input/output probes 702a and 702b are connected to center conductors of the input/output terminals 701a and 701b by soldering or the like.
The dielectric resonance element 703 has a cylindrical shape, is placed substantially at a center of the casing 704, and is pinched between a bottom surface 710 of the casing 704 and the metallic cover 705, with its upper flat surface 709 placed on the metallic cover 705 and its lower flat surface 712 placed on the bottom surface 710. The casing 704 and the metallic cover 705 are fixed on each other by connecting screws 706 to improve the degree of contact for connection between the lower flat surface 712 of the dielectric resonance element 703 and the bottom surface 710 of the casing 704 and the degree of contact for connection between the upper flat surface 709 of the dielectric resonance element 703 and the metallic cover 705 and to improve the reliability of connection between the casing 704 and the metallic cover 705 so that the discontinuity of current flowing through the connecting portions is reduced.
An inner hole 711 is formed in the cylindrical dielectric resonance element 703. The frequency adjusting screw 707 connected to the casing 704 is inserted in the inner hole 711 in which electric force lines 801 are concentrated to change the resonance frequency of the dielectric resonator. A signal input to the input/output terminal 701a is transferred by electromagnetic coupling between the input/output probe 702a and the dielectric resonance element 703 and electromagnetic coupling between the dielectric resonance element 703 and the input/output probe 702b to be output through the input/output terminal 701b. Thus, this dielectric resonator operates as a TM010 mode dielectric resonator (e.g., see Japanese Patent Publication No. 63-22727, Japanese Patent Publication No. 63-22728, and Japanese Patent Publication No. 63-22729).
In the above-described arrangement, however, a gap occurs between the dielectric resonance element 703 and the metallic casing 704 because of the difference between the linear expansion coefficients thereof when the ambient temperature changes. Considerable changes are thereby caused in the resonance frequency and the Q-value. It is, therefore, difficult to realize a stable resonator and filter. An arrangement has been proposed in which the casing 704 is formed of the same dielectric material as that of the dielectric resonance element 703 to absorb the difference between the linear expansion coefficients of the dielectric resonance element 703 and the metallic casing 704, and in which an electroconductive film is provided on the inner wall (see the above-mentioned patent documents 1 and 3). However, if the casing 704 and the dielectric resonance element 703 are formed of the same dielectric material, the degree of difficulty in manufacturing and the manufacturing cost are increased.
Further, even if a material of a comparatively high conductivity is used as the electroconductive film provided on the inner wall, the conductance of the electroconductive film is lower than that of the metallic casing 704 and the influence on the performance of the resonator of the loss due to the current flowing through the electroconductive film is considerably large, so that the Q-value representing the performance of the resonator is reduced. For this reason, it is difficult to realize a high-performance dielectric resonator and a high-performance filter.
In view of the above-described problems, an object of the present invention is to provide a dielectric filter capable of operating with stability even when a change in temperature occurs, and a method of supporting a dielectric resonance element of the dielectric filter.